As will be appreciated by those skilled in the art, it is desirable to reduce the dark current output of a photodetector to improve the signal-to-noise ratio. In particular, it is known that the detection of long wavelength and low background infrared radiation is difficult due to the presence of a relatively large dark current. In addition to a reduction of the signal-to-noise ratio which reduces the sensitivity of the detector, high dark current also limits dynamic range and increases power dissipation. Power dissipation of semiconductor photodetectors is of considerable concern in the fabrication of focal plane arrays, since excessive heating requires additional device cooling.
Semiconductor photodetectors having multiple quantum wells in a superlattice structure are known in the art. A superlattice is typically fabricated using molecular beam epitaxy or metalorganic chemical vapor deposition to form a multilayered heterojunction structure. The thickness of each active layer is reduced to the order of carrier de Broglie wavelength such that two dimensional quantization occurs, resulting in a series of discrete energy levels. For example, a typical superlattice photodetector includes a plurality of alternating gallium arsenide (GaAs) and aluminum gallium arsenide (AlGaAs) layers. Each period of the superlattice comprises one GaAs layer and one AlGaAs layer. The GaAs layers are heavily doped n-type and comprise the quantum well layers which are interposed between AlGaAs barrier layers. The conduction band edge of the barrier layer material is above that of the conduction band edge of the quantum well layers, forming periodic quantum wells. The height of the energy barrier of the barrier layers can be varied by changing the ratio of aluminum-to-gallium to confine electrons at a selected energy level in the quantum wells.
In order to reduce thermionic emission of electrons from the quantum wells, superlattice devices of this nature are operated at temperatures based on a selected detection wavelength. An electrical bias applied perpendicular to the alternating barrier and quantum well layers in the absence of illumination produces a low current known as the dark current which results from quantum mechanical tunneling of electrons through the potential barriers of the barrier layers. However, when the superlattice is illuminated by photons of the appropriate energy, electrons are excited out of the quantum wells in response to the radiation by transitions between energy levels. These photoexcited electrons increase the conductivity of the device. Thus, it will be appreciated that these devices are in effect photoconductors and that a signal can be derived which is representative of the detected radiation.
One such device is disclosed in European Patent No. 275-150-A, wherein a photodetector having a superlattice defining multiple quantum wells is provided for infrared radiation detection. In this device, electrons in the quantum wells have two bound states. Incident infrared radiation produces intersub-band absorption between the ground state and the excited state. The applied bias, the height of the potential energy barriers of the barrier layers, and the spacing of the energy states in the quantum well layers are configured such that electrons in the excited state have a high tunneling probability. A signal current results from tunneling of the photoexcited electrons through the potential barriers of the barrier layers. In one embodiment, energy levels of neighboring wells are matched to optimize tunneling of photoexcited electrons while inhibiting dark current tunneling.
Other superlattice photodetectors have been designed which do not rely on photoexcited tunneling for the signal current. More specifically, it is known that quantum well structures have finite barrier heights and that permissible energy states exist above the potential barrier of the barrier layers, i.e., in the continuum state of the superlattice. B. F. Levine and others describe a photodetector of this type in an article entitled "High-Detectivity D.sup.* =10.sup.10 cm .sqroot.Hz/W GaAs/AlGaAs Multiquantum Well .lambda.=8.3 .mu.m Infrared Detector," App. L. Phys. Lett., 53(4), 25 July 1988. The detector comprises a 50 period GaAs/AlGaAs superlattice positioned between contact layers grown on a semi-insulating GaAs substrate. One advantage of these devices is the ability to control peak absorption wavelength by varying quantum well layer dimensions and barrier layer composition and thickness. The quantum wells contain a single bound state. By photoexciting the quantum well electrons into the continuum while the superlattice is appropriately biased, electrons travel above the superlattice potential barriers toward the collector, rather than through the barriers by quantum mechanical tunneling. Assuming an adequate mean-free path, the photoexcited carriers produce a signal representative of photon absorption in the quantum well layers.
Although multiple quantum well structures provide higher absorption efficiencies than single-well devices, a larger bias voltage is also required. This, in turn, increases the dark current produced by conventional superlattice photodetectors. The thermionic emission component of the dark current can be effectively minimized by operating these devices at low temperatures. However, it is known that the tunneling current, which is increased by sequential resonant effects and electron hopping, is the major component of the dark current. As stated, in applications requiring the detection of long wavelength and low background infrared radiation, the dark current is a significant problem in the operation of conventional multiquantum well photodetectors. Therefore, it is desirable to reduce the tunneling component of the dark current to increase the signal-to-noise ratio.
The solution proposed by others to reduce dark current in these devices is to increase the thickness of each of the barrier layers of the superlattice. Since photoconduction is not achieved through tunneling, thin barriers are not necessary from the standpoint of optimizing tunneling current. More specifically, in the aforementioned photodetector described by Levine and others, barrier layers of AlGaAs 300 angstroms in thickness and GaAs quantum well layers 40 angstroms in thickness were arranged to form a 50 period superlattice. By increasing the barrier width from 140 to 300 angstroms and the barrier height from 160 mV to 250 mV, the dark current was reduced by several orders of magnitude. This reduction in dark current resulted from a decrease in electron tunneling through the thick barrier layers. However, this method of decreasing the dark current suffers from several serious limitations.
Photodetector performance is based primarily on the quantum efficiency of the device, the response time and the sensitivity of the device. Although increasing the thickness of the superlattice barrier layers reduces dark current, it also limits the quantum efficiency of the detector. It will be appreciated that the mean-free path of electrons through the superlattice in a selected material system is essentially established by the bias voltage. Thus, if barrier layer thicknesses are substantially increased, as suggested, to significantly reduce the dark current, the periodicity number of the superlattice must be reduced to prevent recombination of the photoexcited electrons in the superlattice. As will be appreciated by those skilled in the art, in order for photoexcited charge carriers to be detected, they must have a mean-free path which is at least equal to the distance through the superlattice. If the mean-free path is less than this minimum distance, the photoexcited electrons will fall in the ground state of quantum wells in the superlattice or be trapped in the blocking or barrier layers prior to reaching the ohmic contact. Thus, either the number of superlattice layers must be decreased, resulting in fewer quantum well layers which reduces quantum efficiency, or the applied bias must be increased which in turn increases the tunneling current. In contrast to this prior art suggestion of increasing the thicknesses of the superlattice barrier layers, in the present invention, tunneling is prevented by a single, thick blocking layer positioned between the superlattice and the positively biased ohmic contact with respect to the other ohmic contact.
It should be noted that in U.S. Pat. No. 4,645,707, a semiconductor device is disclosed which includes two superlattices separated by a centrally disposed barrier layer which has a lower transmission coefficient than the barrier layers of the superlattices. It is stated that the central barrier layer is thicker than the barrier layers of the superlattices. The semiconductor device exhibits negative differential conductance due to voltage dependent discontinuities between energy minibands of the two superlattices. The device is a tunneling current device and is not a photodetector.
Accordingly, it is a primary object of the present invention to provide a multiquantum well photodetector having a low dark current.
Another object of the present invention is to provide a low dark current multiquantum well photodetector in which tunneling current is reduced or eliminated by means other than by increasing superlattice barrier layer thicknesses.
It is still a further object of the present invention to provide a multiquantum well photodetector which is particularly efficient in the detection of long wavelength and low background infrared radiation with virtually no power dissipation.
Still another object of the present invention is to provide a low dark current multiquantum well photodetector for use in infrared detector focal plane arrays.